In access control systems, e.g. pay TV systems, square, or more general, d-th roots modulo X are used where X is a composite number having at least two large prime factors. Typically the length of such a number X (denoted .vertline.X.vertline.) is 64 bytes.
If a sender, e.g. a smart-card, communicates with a receiver only few data are transmitted in order to save time. But this results in an increased number of computation operations in the sender and/or receiver.